Mail off first article on beast (6)
I believe the answer is:
animal
'beast' is the definition.
(a beast is an animal)
'mail off first article' is the wordplay.
'off' is an anagram indicator.
'first' means one lot of letters go next to another.
'article' becomes 'an' (English indefinite article).
'mail' anagrammed gives 'imal'.
'imal' after 'an' is 'ANIMAL'.
'on' acts as a link.
(Other definitions for animal that I've seen before include "swine" , "Uncivilised person" , "Beastly" , "It shares the world with vegetable and mineral" , "Mammal" .)